The present invention relates to an image forming apparatus capable of developing operation using, for example, two developing agents, and more specifically to an image forming apparatus in which two types of two-component developing agents are selectively used for selective developing operations with different effects.
Development of color versions of image forming apparatuses, such as copying machines, have recently been in progress. For example, two-color copying machines have been developed for practical use in which color images are developed by superposing a developing agent of a color other than black on a black developing agent.
In one such conventional two-color copying machine, a copy image representing black regions of an original image is first developed by a black developing agent and formed on a copy paper sheet. After copying, the sheet is temporarily removed from the machine. Then, an operator feeds the removed sheet again into the machine. In the machine, the sheet, with the black toner image previously formed thereon, undergoes another cycle of copying operation, in which a copy image representing red regions of the original image is developed by a red developing agent and formed on the sheet. The sheet, carrying thereon the complex image formed on the two developing agents of different colors, is finally discharged from the machine. Thus, the copying operation as an entire sequence is completed.
In the prior art two-color copying machines of this type, two developing units are required as developing means for two-color developing. High-speed copying operation should, however, require the use of magnet rolls with greater diameters in the developing units. Accordingly, it would be difficult to furnish the copying machines with two developing units each.